<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>圆形
    </title>
</head>
<body>

<style>
    div{
        width: 200px;
        height: 200px;
        border: 5px solid red;
        border-radius: 50%;
    }
</style>
<div>

</div>
</body>
</html>